Giant Clams are hermaphrodites (having both male &
female organs). However, they reproduce sexually,
not by self-fertilization.

A 'Spawning Induced Substance' is released into the
water
from the outlet siphon. Detection of SIS through the
inlet siphon stimulates other giant clams to swell
their mantle,
contract adductor muscles & vigorously release eggs
& sperm.

Since Tridacnas
cannot move, this broadcast spawning
('synchronized release' of sperm & eggs)
reduces the burden
of finding a compatible mate, simultaneously doubles
the number of offspring produced, and ensures that
new
gene combinations are passed to further generations.